Various high-frequency filters using resonators having resonant frequencies and anti-resonant frequencies have hitherto been devised. For example, Patent Documents 1 and 2 describe high-frequency filters capable of varying filter characteristics, such as bandpass characteristics and attenuation characteristics, using piezoelectric resonators.
The high-frequency filters described in Patent Documents 1 and 2 each have a circuit configuration in which a variable capacitor is connected in series or in parallel to the piezoelectric resonator. Each of the high-frequency filters described in Patent Documents 1 and 2 varies at least one of the resonant frequency and the anti-resonant frequency of a circuit composed of the piezoelectric resonator and the variable capacitor by adjusting the capacitance of the variable capacitor in order to adjust the filter characteristics. The filter characteristics include the bandpass characteristics and the attenuation characteristics.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2009-130831
Patent Document 2: Japanese Patent No. 4053504